Dimethyl-Aluminium Complexes Bearing Naphthyl-Substituted Pyridine-Alkylamides as Pro-Initiators for the Efficient ROP of ε-Caprolactone

Three sterically-enhanced 2-imino-6-(1-naphthyl)pyridines, 2-{CMe=N(Ar)}-6(1-C10H7)C5H3N [Ar = 2,6-i-Pr2C6H3 (L1dipp), 2,4,6-i-Pr3C6H2 (L1tripp), 4-Br-2,6-i-Pr2C6H2 (L1Brdipp)], differing only in the electronic properties of the N-aryl group, have been prepared in high yield by the condensation reaction of 2-{CMe=O}-6-(1-C10H7)C5H3N with the corresponding aniline. Treatment of L1dipp, L1tripp and L1Brdipp with two equivalents of AlMe3 at elevated temperature affords the distorted tetrahedral 2-(amido-prop-2-yl)-6-(1-naphthyl)pyridine aluminum dimethyl complexes, [2-{CMe2N(Ar)}6-(1-C10H7)C5H3N]AlMe2 [Ar = 2,6-i-Pr2C6H3 (1a), 2,4,6-i-Pr3C6H2 (1b), 4-Br-2,6-i-Pr2C6H2 (1c)], in good yield. The X-ray structures of 1a–1c reveal that complexation has resulted in concomitant C–C bond formation via methyl migration from aluminum to the corresponding imino carbon in L1aryl; in solution, the restricted rotation of the pendant naphthyl group in 1 confers inequivalent methyl ligand environments. The ring opening polymerization of ε-caprolactone employing 1, in the presence of benzyl alcohol, proceeded efficiently at 30 °C producing polymers of narrow molecular weight distribution with the catalytic activities dependent on the nature of the substituent located at the OPEN ACCESS Catalysts 2015, 5 1426 4-position of the N-aryl group with the most electron donating i-Pr derivative exhibiting the highest activity (1b > 1a > 1c); at 50 °C 1b mediates 100% conversion of the monomer to polycaprolactone (poly(CL)) in one hour. In addition to 1a, 1b and 1c, the single crystal X-ray structures are reported for L1dipp and L1tripp.

We have been attracted by the intriguing properties displayed by pyridylimine-containing ligands including their redox activity [43,44] and their capacity to undergo nucleophilic attack on the imine carbon [45][46][47][48][49][50] and the pyridine ring [51].Indeed, the thermally-induced migration of an Al-alkyl to the imino carbon of a coordinated N,N-pyridylimine represents a powerful tool for transforming a pyridylimine into a pyridyl-alkylamide (and pyridyl-alkylamine on hydrolysis [45,52]).Moreover, functionalizing the 6-position of the pyridine moiety of the pyridylimine with an aryl or 1-naphthyl group presents a versatile set of substrates for studying ortho vs. peri-palladation reactions during the formation of N,N,C-chelates [53,54].Likewise, N,N-pyridyl-alkylamides and their 6-aryl and -naphthyl derivatives have also been attracting attention due, in large measure, to their emergence as supports for exceptionally active N,N- [45,55,56] and N,N,C-bound group 4 olefin polymerization catalysts [57][58][59][60].
In this article we are concerned with exploiting the reactivity of the imino unit in a series of sterically-hindered 6-naphthyl-substituted pyridyl-ketimines, L1 (R = H, i-Pr, Br; see Figure 1), towards nucleophilic attack with a view to forming aluminum methyl complexes bound by a series of electronically distinct pyridyl-alkylamides, L2 (R = H, i-Pr, Br).The potential of the naphthyl group to undergo peri/ortho C-H sp 2 -activation during complexation or simply provide a bulky substituent with restricted mobility, presents a further point of interest.The resultant aluminum methyl complexes will be screened, in the presence of benzyl alcohol, for the ROP of ε-CL and any electronic effect imparted by the 4-R-substituted N-aryl group on the catalytic performance, investigated.

Scheme 1. Reagents and conditions:
In addition, single-crystal X-ray diffraction studies have been performed on L1dipp and L1tripp.Typically, crystals suitable for the structural determinations were grown by slow evaporation of methanol solutions of the compound.A view of L1tripp is depicted in Figure 2 (see also Figures S1 and S2); selected bond distances and angles are listed for both L1dipp and L1tripp in Table 1.The structures are similar with a central pyridine ring substituted at its 6-position by a 1-naphthyl group and at the 2-position by an imine unit [C(6)-N( 2    Compounds L1dipp, L1tripp and L1Brdipp all display peaks corresponding to the protonated molecular ions in their ESI mass spectra while their IR spectra reveal characteristic ν(CN)imine bands ca.1642 cm −1 .Further support for imine formation is provided by the 1 H NMR spectra which show signals for the ketimine methyl protons at ca. δ 2.2 (L1dipp-L1Brdipp) (see Figures S3-S5); the imino carbons are seen at ca. δ 160 in their 13 C{ 1 H} NMR spectra (see Figures S6-S8).
Single crystals of 1a, 1b, and 1c suitable for X-ray determination were grown by slow cooling of acetonitrile solutions of each complex that had been previously brought to reflux.A perspective view of representative 1c is shown in Figure 3 (see also Figures S9-S11); selected bond distances and angles for all three structures are collected in Table 2.In each structure an aluminum center is bound by a monoanionic N,N-pyridine-alkylamide chelate along with two methyl ligands to complete a distorted tetrahedral geometry.The bite angle for the bidentate ligand ranges from 83.9(2)° up to 85.4(2)°, while the X-M-X angles between monodentate methyl ligands are closer to tetrahedral, ranging from 111.1(3)° up to 113.5(3)°.The presence of the gem-dimethyl group within the bidentate ligand backbone in 1a, 1b and 1c confirms the successful methyl migration to the imine unit in L1aryl and an associated elongation of the C(8)-N(2) bond by ca.0.2 Å is observed.The five-membered chelate rings display some variation in the degree of puckering between structures which is best exemplified by the C(7)-C(8)-N(2)-Al (1) torsion angles that vary between 1.3° (1b) and 27.1° (1c).Of the two Al-N bonds present in each structure, the one involving the amide [N(2)] is the shorter consistent with the ionic contribution to the bonding; no clear effects caused by the change in N-aryl 4-R group on the Al-N(2)amide distances are apparent between structures.In comparison with L1dipp/tripp the pendant naphthyl groups in 1a-1c are tilted more towards orthogonality with respect to the adjacent pyridine unit [tors.: N(1)-C(3)-C(23)-C(32) 75.3° (1a), 69.5° (1b), 65.5° (1c)], with the result that one of the two Al-Me groups faces the fused diarene unit.Similar alkyl migration reactions mediated by alkylaluminums have been reported elsewhere [45][46][47][48][49][50][51][52], and indeed computational studies suggest a radical pathway for the transformation [44].The closest comparators to

Complex
Bond Angles (deg)  Support for the solid state structures of 1a-1c being maintained in solution is provided by the inequivalency of the backbone N-CMe A Me B methyl protons in their 1 H NMR spectra (recorded in C6D6 at ambient temperature), which is accompanied by two distinct septets for the Ar-o-CHMe2 protons and four separate doublets for the Ar-CHMe2 protons (see Figures 4, S12-S14).This inequivalency can be attributed to the restricted rotation of the naphthyl group; for purposes of

Polymerization Results
Complexes 1a-1c were all screened as pro-initiators for the ring-opening polymerization of ε-CL.Typically, 1a-1c were treated with one equivalent of benzyl alcohol in toluene prior to the addition of the ε-CL (250 equivalents) and the start of the run; all systems were evaluated at 30 °C and selected examples at 50 °C (Scheme 2).The polymerization runs were monitored at 30 min intervals by 1 H NMR spectroscopy to determine the ε-CL to poly(CL) conversion.In addition the resultant polycaprolactone polymers were analyzed by size exclusion chromatography (SEC).Scheme 2. Catalytic evaluation of 1/PhCH2OH for the ROP of ε-CL.
The results of the catalytic screening are collected in Table 3 (entries 1-14).At 30 °C, all the systems display moderate to good activity [5] for the polymerization of ε-CL with para-i-Pr-containing 1b/PhCH2OH being the most active reaching 80% conversion to poly(CL) after 2 h (entry 8).The para-Br-containing 1c/PhCH2OH and para-H-containing 1a/PhCH2OH proved less active reaching only 38% (entry 12) and 59% (entry 4) over the same time period, respectively.In the case of 1b, full conversion was reached after only one hour when the run was performed at 50 °C (entry 14).The dispersities of the polyesters were narrow with unimodal characteristics (Đ = Mw/Mn = 1.09 to 1.65) with the runs for 1b performed at the higher temperatures (entries 13,14) falling at the higher end of the range.All three systems display an approximately linear relationship between number-average molecular weight (Mn) and monomer conversion which, coupled with the relatively low values of the dispersities, implies controlled polymerizations (Figure 5).The apparent broadening of the dispersities at higher temperature would suggest the onset of some transesterification reactions albeit minimal.In general, there was reasonable agreement between the observed and calculated molecular weights, again indicating good control over the polymerization process.The MALDI-ToF spectrum of a sample of PCL obtained from 1a/PhCH2OH reveals a series of major peaks separated by a caprolactone unit (114 g• mol −1 ) corresponding to linear [H-(CL)n-OBn]• Na + cations (see Figure S18).This would suggest a coordination-insertion mechanism, as seen by others, is operational making use of an Al-OCH2Ph group in the polymerization [1][2][3][4][5].Moreover, the 1 H NMR spectrum of a sample of PCL obtained using the same catalyst showed a peak at 5.12 ppm corresponding to the benzyl ester end group and a signal at 3.65 ppm to the hydroxymethylene (-CH2OH) end group.Unexpectedly, the same MALDI-ToF spectrum also displayed minor peaks that could be assigned to linear [H-(CL)n-OH]•Na + cations, the likely product of hydrolysis under ionization conditions.
It is apparent from the above results that the electron-donating capacity of the N-aryl 4-R group of the N,N-pyridine-alkylamide ancillary ligand influences the catalytic activity with para-isopropyl-containing 1b showing the highest activity; unexpectedly, however, the more electron-withdrawing para-bromo 1c is more active than the para-hydrogen 1a.It has been proposed previously that the overall rate of the polymerization depends on a combination of factors including the Lewis acidity of the metal center and the alkoxide nucleophilicity [17].For the systems described in this work it would appear that there is a delicate balance between these factors with the superior alkoxide nucleophilicity in 1b/PhCH2OH likely to more influential in this case; similar electron donating rate enhancements have been noted elsewhere [17,32,67].Undoubtedly the rearrangements that ensue in the final ring opening of the monomer further contribute to the overall polymerization rate.
Given the inequivalence of the aluminum methyl ligands of 1a-1c in the 1 H NMR spectra (vide supra), it was of interest to monitor any site selectivity in the reaction with benzyl alcohol.Unfortunately, on treatment of 1a with one equivalent of benzyl alcohol in C6D6 a mixture of products was formed as evidenced by a series of the peaks in 1 H NMR spectrum in the region characteristic of the Al-OCH2Ph protons (between δ 4.9-5.4)[19,68].This observation would suggest that the presence of ε-CL is crucial for generating the single active Al-benzyloxide species.

General Procedures
All reactions, unless otherwise stated, were carried out under an atmosphere of dry, oxygen-free nitrogen, using standard Schlenk techniques.Solvents were distilled under nitrogen from appropriate drying agents and degassed prior to use [69].NMR spectra were recorded on a Bruker DRX400 spectrometer (Coventry, UK) at 400.13 ( 1 H) and 100.61MHz ( 13 C) at ambient temperature unless otherwise stated; chemical shifts (ppm) are referred to the residual protic solvent peaks and coupling constants are expressed in hertz (Hz).The electrospray ionization (ESI) and the fast atom bombardment (FAB) mass spectra were recorded using a micromass Quattro LC mass spectrometer (Manchester, UK) and a Kratos Concept spectrometer (Manchester, UK) with methanol or nitrophenyloctylether as the matrix, respectively.High-resolution FAB mass spectra were recorded on Kratos Concept spectrometer (xenon gas, 7 kV) with NBA as matrix.The MALDI-ToF mass spectrum of PCL was obtained with an ABI Voyager-DE™ STR (Warrington, UK), BioSpectrometry™ Workstation (Warrington, UK), Serial No.: 4364, using a nitrogen laser source (337 nm, delay time 500 ns) in reflector mode with a positive acceleration voltage of 25 kV.The infrared spectra were recorded on a Perkin-Elmer Spectrum One FT-IR spectrometer (Buckinghamshire, UK) on solid samples.Melting points (mp) were measured on a Gallenkamp melting point apparatus (model MFB-595, Loughborough, UK) in open capillary tubes and were uncorrected.Elemental analyses were performed on a Carlo Erba CE1108 instrument at the Department of Chemistry, London Metropolitan University (London, UK).

Catalytic Evaluation
A typical polymerization procedure is as follows (Table 3).An oven-dried Schlenk vessel equipped with stirrer bar was loaded in the glove box.The aluminum pro-initiator 1 (0.04 mmol) was introduced, followed by 15 mL of a 0.0027 M solution of benzyl alcohol (0.04 mmol, 1 eq.) in toluene.The mixture was stirred for 10 min at room temperature and then placed in an oil bath pre-heated to the desired temperature.ε-CL (1.1 mL, 10.0 mmol, 250 eq.) was added and the mixture allowed to stir for the designated time period.A small aliquot (0.2 mL) of the reaction mixture was removed at selected time intervals, treated with a few drops of methanol and the residue analyzed by 1 H NMR spectroscopy (to determine monomer conversion) and by SEC (to determine Mn and Đ).All conversion measurements were repeated in triplicate.

Size Exclusion Chromatography
Size Exclusion Chromatography analyses of the samples were performed on an EcoSEC semi-micro GPC system from Tosoh (Minato-ku, TKY, Japan) equipped with a dual flow refractive index detector and a UV detector.The samples were analyzed in THF at 30 °C using a flow rate of 1 mL• min −1 .All polymers were injected at a concentration of 1 mg• mL −1 in THF, after filtration through a 0.45 μm pore-size membrane.Separation was performed with a guard column and three PL gel 5 μm MIXED-C (7 µm, 300 × 7.5 mm).The average molar masses (number-average molar mass Mn and weight-average molar mass Mw) and the dispersity (Đ = Mw/Mn) were derived from the RI signal by a calibration curve based on poly (styrene) standards.The calibration was constructed with narrow molecular weight standards from 580 g/mol to 3,053,000 g/mol.A third-degree polynomial regression was applied.WinGPC software (PSS Polymer Standards Service, Mainz, Germany) was used for data collection and calculation.The Mn values of the PCLs were corrected with a factor of 0.56 to account for the difference in hydrodynamic volumes with polystyrene [65,66].

Crystallographic Studies
Data for L1dipp, L1tripp, 1a, 1b and 1c were collected on a Bruker APEX 2000 CCD diffractometer.Details of data collection, refinement and crystal data are listed in Table 4.The data were corrected for Lorentz and polarization effects and empirical absorption corrections applied.Structure solution by direct methods and structure refinement based on full-matrix least-squares on F 2 employed SHELXTL version 6.10 [75].Hydrogen atoms were included in calculated positions (C-H = 0.95-1.00Å) riding on the bonded atom with isotropic displacement parameters set to 1.5 Ueq(C) for methyl H atoms and 1.2 Ueq(C) for all other H atoms.All non-H atoms were refined with anisotropic displacement parameters.Disordered solvent was omitted using the SQUEEZE option in PLATON for 1c [76].

Figure 2 .
Figure 2. Molecular structure of L1tripp with atom labeling scheme; all hydrogen atoms have been omitted for clarity.

Figure 3 .
Figure 3. Molecular structure of 1c with atom labeling scheme.All hydrogen atoms have been omitted for clarity.

Table 3 .
Ring opening polymerization of ε-CL initiated by 1/PhCH2OH catalyst systems a .

Table 4 .
Crystallographic and data processing parameters for L1dipp